OncoMed has been informed by the participating clinical sites of recent on-target mild-to-moderate bone-related adverse events for the two programs. To date, bone-related adverse events have been observed in 8 of 63 (13%) patients treated with vantictumab and 2 of 41 (5%) patients treated with Fzd8-Fc. After careful analysis of the recent mild-to-moderate adverse event incidents, OncoMed has halted enrollment and dosing in the Phase 1 studies for both programs as a precautionary measure. OncoMed, in conjunction with its academic bone expert advisors and study investigators, continues to analyze the clinical data in order to submit amended protocols to the U.S. Food and Drug Administration (FDA) and subsequently to the clinical study sites.

The amendments for the Phase 1b combination trials will include 1) modified dosing regimens, such as lower and less frequent dosing, 2) updated risk mitigation measures, such as increased monitoring and bone protection strategies, and 3) modified enrollment criteria. Enrollment and dosing of new patients is expected to resume once amendments go through the process of review by the FDA and approval by the study sites' institutional review boards (IRBs).

In parallel, the company intends to continue existing or modified dosing of those patients in the completed single-agent Phase 1a clinical trials for both vantictumab and Fzd8-Fc who have remained on treatment with the investigational agent for extended periods of time without disease progression and without significant drug-related adverse events.

OncoMed has notified the FDA of these actions.

"With patient safety as our first priority, we aim to fully understand these events, continue to conduct a thorough analysis of data in hand, and work closely with the investigators and the FDA to determine the best path forward," stated Paul J. Hastings, Chairman and Chief Executive Officer of OncoMed.

About OncoMed Pharmaceuticals

OncoMed Pharmaceuticals is a clinical-stage company focused on discovering and developing novel therapeutics targeting cancer stem cells (CSCs). OncoMed has five anti-cancer product candidates in clinical development, including demcizumab (anti-DLL4, OMP-21M18), tarextumab (anti-Notch 2/3, OMP-59R5), anti-Notch1 (OMP-52M51), vantictumab (anti-Fzd7, OMP-18R5), and Fzd8-Fc (OMP-54F28), which target key cancer stem cell signaling pathways including Notch and Wnt. OncoMed has two other antibodies in preclinical development, anti-DLL4/anti-VEGF bispecific (OMP-305B83) and anti-RSPO3 (OMP-131R10), with Investigational New Drug filings planned for late 2014 or early 2015. OncoMed is also pursuing discovery of additional novel anti-CSC product candidates. OncoMed has formed strategic alliances with Celgene Corporation, Bayer Pharma AG and GlaxoSmithKline. Additional information can be found at the company's website: www.oncomed.com.

Forward-Looking Statements

To the extent that statements contained in this press release are not descriptions of historical facts regarding OncoMed Pharmaceuticals, they are forward-looking statements reflecting the current beliefs and expectations of management made pursuant to the safe harbor provisions of the Private Securities Litigation Reform Act of 1995, including OncoMed's expectations regarding the ability of OncoMed to advance vantictumab and OMP-54F28 in clinical development; the submission and content of amended protocols for the vantictumab and OMP-54F28 Phase 1 trials; the acceptability of OncoMed's amended vantictumab and OMP-54F28 Phase 1 protocols by the Food and Drug Administration and institutional review boards; the resumption of enrollment and dosing of new patients in the Phase 1b trials for vantictumab and OMP-54F28; the continuation of dosing of patients in the Phase 1a vantictumab and OMP-54F28 trials; and the timing of Investigational New Drug filings for OncoMed's anti-DLL4/anti-VEGF bispecific and anti-RSPO3 antibodies. Such forward-looking statements involve substantial risks and uncertainties that could cause OncoMed's clinical development programs, future results, performance or achievements to differ significantly from those expressed or implied by the forward-looking statements. Such risks and uncertainties include, among others, the uncertainties inherent in the preclinical and clinical development process; the risks and uncertainties of the regulatory approval process; OncoMed's dependence on its collaboration partners, including Celgene, GSK and Bayer, for the funding of its partnered programs; OncoMed's ability to raise additional capital to support the development of its unpartnered programs; OncoMed's dependence on the development and marketing efforts of its partners for the commercial success of its partnered product candidates; OncoMed's reliance on third parties to conduct certain preclinical studies and all of its clinical trials; OncoMed's reliance on single source third-party contract manufacturing organizations to manufacture and supply its product candidates; OncoMed's ability to validate, develop and obtain regulatory approval for companion diagnostics; OncoMed's ability to achieve market acceptance and commercial success of its product candidates once regulatory approval is achieved; OncoMed's ability to discover, develop and commercialize additional product candidates; the ability of competitors to discover, develop or commercialize competing products more quickly or more successfully; OncoMed's dependence on its Chairman and Chief Executive Officer, its Chief Scientific Officer, its Chief Medical Officer and other key executives; risk of third party claims alleging infringement of patents and proprietary rights or seeking to invalidate OncoMed's patents or proprietary rights; and the ability of OncoMed's proprietary rights to protect its technologies and product candidates. OncoMed undertakes no obligation to update or revise any forward-looking statements. For a further description of the risks and uncertainties that could cause actual results to differ from those expressed in these forward-looking statements, as well as risks relating to OncoMed's business in general, see OncoMed's Annual Report on Form 10-K for the fiscal year ended December 31, 2013, filed with the Securities and Exchange Commission (SEC) on March 18, 2014, and OncoMed's Quarterly Report on Form 10-Q for the fiscal quarter ended March 31, 2014, filed with the SEC on May 8, 2014.